The present invention is concerned with a method of concentrating a low-concentration element or group of elements in a solution with a view to their quantitative analysis, for example by emission spectrometry with an inductive plasma excitation source.
In an apparatus for analysis by emission plasma-spectrometry of an element in solution, the solution is supplied to a nebulizer where it is transformed into an aerosol. The aerosol is then introduced into an inductive plasma source where it is excited and emits radiation characteristic of the elements contained in the solution. This radiation is dispersed into spectra in a spectrometer, and a spectral line characteristic of each element to be quantitatively analysed is measured. The intensity measured is proportional to the concentration of the element.
Practical difficulties are encountered in such an analysis technique when the solution studied, or the sample put in solution in advance, contains a high-concentration element or group of elements, often termed "matrix", when other extremely low-concentration elements are the object of interest. This is the case, for example, when quantitative analysis is required of elements such as lead, iron or copper in seawater, in which the concentration of these elements as traces can be a few ppt (part per trillion) whereas the sodium chloride content is of the order of 3%.
This is also the case for the quantitative analysis of elements such as uranium in a rock, in which high concentrations of silicon, aluminum, iron, calcium, magnesium, sodium and potassium are found.
In such instances, the emission of lines is affected by the chemical composition of the matrix. As a result, the spectral background produced by the matrix varies according to the type of sample to be analysed. In such circumstances, with a very low-concentration element, the signal emitted which is recorded in the form of a peak in a traced curve, does not stand out distinctly and clearly but often remains confused with the background. In other words, the signal to background ratio is not sufficiently high to be measured reliably. On the other hand, one or several elements of the matrix can cause spectral interference, for example, interference by iron and calcium in the quantitative analysis of uranium in a rock.
It therefore became necessary to discover a method of concentrating the element in question and the ion exchange technique was used to achieve this. But in current practice this technique is generally used to separate one element concerned and not to enrich it. For separation by ion exchange, a glass column 1 to 3 cm in diameter is generally used, filled with resin up to heights of 10 to 50 cm; a significant volume of solution is made to pass through the column for a first stage of fixing the element in question; for the following elution stage, a significant volume of solution of an elution agent is made to pass through. At the end of the procedure, the element concerned is separated but is found in a significant volume of concentration. To concentrate it, this solution has to be evaporated until dry and the residue dissolved with a minimum volume of solution. The residue can contain salts introduced by the elution agent, for example NH.sub.4 Cl, which can extend and complicate the evaporation stage. All these procedures make continuous or sequential analyses practically impossible.